Low prf pulse doppler radar having low speed ground moving target rejection

ABSTRACT

An AMTI radar in which the transmitted pulse train is comprised of multiple PRFs which are sequentially switched in a predetermined order during a target dwell time. A digital MTI comb or notch filter having a controlled reject and pass band characteristic for each PRF is utilized such that when operating in one of the lower PRFs, a wide notch characteristic is used for a time after radar pulse transmission during the interval which will provide unambiguous range for the highest PRF, i.e. during the interpulse interval of the highest PRF. Then a narrower notch characteristic is used for the remainder of the interpulse period of the instant PRF. A composite notch filter characteristic is thus obtained for all of the PRFs during a specific target dwell time which will reject not only ground clutter, but also low speed ground moving targets while passing relatively high speed airborne target returns during at least one of the PRFs.

United States Patent 1 [111 3,787,851

Hughes 1 Jan. 22, 1974 LOW PRF PULSE DOPPLER RADAR PrimaryExaminerBenjamin A. Borchelt HAVING LOW SPEED GROUND MOVING AssistantExaminer-G. E. Montone I TARGET REJECTION Attorney, Agent, or FirmF. H.Henson et al.

[75] Inventor: aodbert B. Hughes, College Park, ABSTRACT An AMT! radarin which the transmitted pulse train is [73] Asslgnee: WestmghmseElectnc corporat'on comprised of multiple PRFs which are sequentiallypmsburghi switched in a predetermined order during a target 2 Filed; Dec13 1971 dwell time. A digital MTI comb or notch filter having acontrolled reject and pass band characteristic for PP 207,253 each PRFis utilized such that when operating in one of the lower PRFs, a widenotch characteristic is used [52] US. Cl. 343/7.7, 343/ 17.1 PF for atime after radar P1115e transmission during the 51 Int. Cl. G01s 9/4'2terval which will Provide unambiguous range for the [58] Field of Search343/71, 17.1 PF, 17.2 highest PRF, during the interpulse interval f hhighest PRF. Then a narrower notch characteristic is [56] References Cid used for the remainder of the interpulse period of the UNITED STATESPATENTS instant PRF. A composite notch filter characteristic is thusobtained for all of the PRFs during a specific tar- 3,63l,489 l2/l97lCooper 343/7.7

. get dwell time which will re ect not only ground cluta g tz z iz ter,but also low speed ground moving targets while passing relatively highspeed airborne target returns during at least one of the PRFs.

11 Claims, 7 Drawing Figures lIS I4 [I0 I (42 24 2e 40 X R I Z 2 DELAY/PRF 2 DELAY /PRF' [TARGET I 22 SIGNALS -ANTENNA I 30 8 l I KI l 36,DIGITAL CLUTTER I NOTCH FILTER l l 34 I I NOTCH WIDTH SWITCHING CONTROLPLURAL PRF NOTCH GENERATOR SWITCHING ,,2()

AND TIME SEQUENCER SELECTOR PATENTED 3,787. 851

sum 1 NT 3 I4 I [I0 I I I 42 I I 24 2s 40 I xMITTR RCVR I I E DELAY /PRF2 DELAY /PRF ITARGET I 22 sIGNALs I I ANTENNA I 30 28 32 I 2 I I I 36DIGITAL CLUTTER I I w NDTGII FILTER I I K I 2 I NOTCH WIDTH SWITCHINGJ18 CONTROL 7 IQ I F IGIA PLURAL PRF NOTCH GENERATOR SWITCHING #20 ANDTIME GEDDEN ER sELEGTDR TARGET I f DwELL TIME IPRF I PRF I PRF I PRF IPRF I I I 2 3 4 5 TINE--- GRDIIND TRANS. GLuTTER RETURN LINE 52 52 44 52NDTGIIED FILTER I (PASS DANDI 52 4G 46 52 52 50 r-I F! AMI I I I I I I II/.\I I I I I I I GROFIJLID TARGET I I HIGH SPEED I AIRBggNE TARGET 2PATEIITEIII IIIH 3.}787, 851

SHEET 3 OF 3 .WUSEM EMNQICT E,E U E II J,QE. NQIQL..- T W F HIGHEST H HPRF I INTERPULSE INTERVAL T USE LNARROW USE WIDE NOTCH II0TcII I USEWIDE NOTCH LOWER/ USE PM USE WIDE NOTCH +MARIIOW NOTCH USE WIDE NOTCH TT T I TRANSMITTER I PULSE 4 MAXIMUM RANG h JFEESENT M ME l /LOW PRFREJECTION(BLIND) g REGION CLEAR REGION 5 L MAXIMUM GE v OF PRIOR YsTEMs-H|GH PRF 0 TARGET DOPPLER FREQUENCY OR VELOCITY VELOCLILW UETTITITYTARGETS TARGETS H65 BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates generally to coherent pulse Dopplerradar systems and more particularly to a low PRF Doppler radar whichexhibits ambiguous Doppler, unambiguous range, commonly referred to asan AMTI system. The purpose of this invention is to reduce all mainbeam, ground return clutter and low speed ground target returns below apredetermined threshold level.

2. Descriptionof the Prior Art Low PRF pulse Dopper (AMTI) radars arecapable of detecting and tracking targets in a downlook operating modewhere the target return is time coincident with heavy ground clutter.This capability is realized by discriminating against stationary groundreturn in the frequency domain while accepting returns from movingtargets whose Doppler frequencies differ from the Doppler frequency ofthe stationary ground clutter. The characteristic feature of such aradar is that the transmitted signal must be coherent with the referencesignal in the receiver. Such systems are generally disclosed in theIntroduction to Radar Systems, by Skolnik, published by McGrawHill(1962).

To be unambiguous, the return pulse reflected from the target must occurbetween the first and second pulse transmitted from the radar. Moreover,in a high speed aircraft radar platform, the clutter return spectrumspreads due to the Doppler shift caused by the relative velocity. Thisclutter spread could be overcome by the use of a relatively high PRF:however, this gives rise to ambiguous range information at long rangesdue to the fact that the radar return signal would be received after asubsequent transmitted pulse. Therefore, in order to provide longunambiguous range operation a relatively low PRF must be utilized. Acharacteristic of such a radar, however, is that it is unable todetermine unambiguously the velocity of detected targets based onDoppler information. This characteristic has been found to bedetrimental when searching for airborne targets where relatively slowermoving surface vehicles are also encountered. A related effect resultingfrom Doppler ambiguous PRF is referred to as blind speeds and has beenovercome in the prior art by means of utilizing staggered pulserepetition frequencies such as disclosed in US. Pat. No. 3,480,953

issued to J. S. Shreve and US. Pat. No. 3,491,360 issued to W.Stoorvogel.

Another solution to the problem is to utilize very wide clutterrejection filters which not only reject stationary ground clutter, butalso the Doppler frequencies associated with low velocity surfacevehicles. Such a system might comprise the teachings of US. Pat. No.3,267,468 issued to K. S. Stull, Jr. This approach, however, has twodirect consequences, one of which is it imposes the need for higher PRFsthan would otherwise be used since a rejection band wide enough fortypical surface traffic would occupy the entire intra-PRF Dopplerinterval and secondly the wide reject band aggravates the low PRF pulseDoppler blind speed problem.

Accordingly, the present invention discloses means for providing desiredsurface vehicle rejection while at the same time extending unambiguousrange information for relatively high speed aircraft targets.

SUMMARY Briefly, the subject invention comprises a low PRF pulse Dopplerradar system including means for providing a plurality of PRFs switchedsequentially in a predetermined order during a target dwell timeAdditionally, the invention includes a controlled digital notched filtercoupled to the receiver portion of the radar whose filter characteristicis varied in time relationship with each PRF. In each filtercharacteristic, there is a reject band of either il or 1 normalizedfrequency units about each PRF harmonic and the notch characteristic isswitched during the PRF sequence so that a wide notch characteristic (il/z frequency unit reject band) is used during the first part of theinterpulse interval of each PRF equalling the period of the selectedhigher or highest PRF utilized and then switching to a narrow notchcharacteristic (1% frequency unit reject band) for the remainder of therespective interpulse interval for all the lower PRFs. A notch switchingtime selector circuit and a notch width switching control circuit arecoupled between the filter and PRF generator for controlling appropriatefilter characteristic in response to the PRF sequence. By making thereject bandil /2 frequency units about each PRF harmonic for short rangeand 1% frequency units for long range while utilizing a plurality ofPRFs, a desired ground target rejection band is provided which providesassured visibility of relatively high speed (large Doppler) airbornetargets in at least one of the PRFs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a block diagramillustrative of the preferred embodiment of the subject invention;

FIG. 1B is a pulse diagram of a plurality of transmitted radar pulseshaving different PRFs transmitted during a target dwell time;

FIG. 2 illustrates the frequency spectrum of the system operating at oneof the plurality of PRFs and the corresponding notched filtercharacteristic utilized therewith;

FIG. 3A is a set of waveforms illustrative of a wide notch comb filtercharacteristic having a reject band of il /2 normalized frequency unitsabout each PRF harmonic;

FIG. 3B is a set of waveform illustrative of a narrow notch comb filtercharacteristic wherein each filter has a reject band of 35% normalizedfrequency units about each PRF harmonic;

FIG. 4 is a. set of waveforms. illustrative of the switched notch filtercharacteristic provided in relation to the PRFs utilized; and

FIG. 5 is a graph illustrative of the range capability of the presentinvention in relation to prior art systems.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawingsand more particularly to FIG. 1A, reference numeral 10 generally refersto the transmitter/receiver section of a typical airborne, low PRFpulsed Doppler radar. As is well known to those skilled in the art sucha radar system is a coherent system in which the transmitted signal iscoherent, that is in phase with the reference signal in the receiver.The

transmitter portion may be comprised of a microwave stabilized lockedoscillator (STALO) not shown triggered by means of a pulse train from aPRF generator 12 controlled by a synchronizer, not shown. The STALOnormally provides an output pulse of carrier frequency f which isapplied to a radiant antenna 14 through a gated amplifier and duplexer,also not shown.

A STALO normally provides a second output of re duced amplitude and afrequency f r f which is applied to a mixer whose other input comprisesthe radar returnpulse 1",, received at the antenna and fed theretothrough the duplexer. The mixer then provides a coherent IF signal ofthe radar return pulse at a frequency f In the present invention the PRFgenerator 12 sequentially generates groups of triggering pulses witheach group respectively having a PRF which is different from the groupwhich precedes it. This pulse train causes the transmitter portionof theradar to generate radar output pulses whose PRF is sequentially variedin a predetermined order, e. g. going from a relatively high PRF to arelatively low PRF during a target dwell time such as shown in FIG. 1B.In the specific illustrative example, pulses of five succesivelydecreasing PRFs are radiated from the radar antenna 14 during the targetdwell time. The target dwell time is defined as the time it takes theantenna 14 to scan the target of interest. The reflected target returnis picked up by the antenna 14, fed through the receiver portion of thetransmitter/- receiver section such as by means of a duplexer and firstmixer, not shown, which is then fed to a digital clutter notched combfilter 16 which has a variable externally controlled filtercharacteristic such as shown in FIGS. 3A and 3B. The filtercharacteristic is selectively varied in passband as well as the rejectband for each PRF as the plural or multiple PRFs are sequenced. Thenotch or reject bandwidth of the filter characteristic is controllablebetween at least two reject bandwidths or notches. This is accomplishedby means of a notch width switching control circuit 18 coupled to anotch switching time selector 20 which is responsive to the PRF sequenceproduced by the PRF generator and sequencer 12. i

The digital clutter notch filter circuit 16 is a two pole recursivefilter having two adjustable feedback constants K1 and K2. An IF signalcorresponding to a target return, having a frequency spectrum for aparticular PRF as shown in FIG. 2, is fed by means ofa signal path 22 toone input of a first and second signal summer 24 and 26, respectively.The second summer 26 receives as its other input the output ofa firsttime delay circuit 28 which has a delay of 1/PRF. The input of the firsttime delay circuit 28 comprises the output of the first summer 24. Thefirst summer 24, on the other hand, receives as its other input theoutput of a third summer 30. One input of summer 30 is the output of thesecond summer 26 fed through a feedback amplifier 32 which has its gainK controlled by means of a control signal feed thereto from the notchwidth switching control circuit 20 over signal elad 34. The other inputto the summer 30 is from a second feedback amplifier 36 whose gain K isalso adjustable by means of a signal coupled from the notch widthswitching control circuit 20 over signal path 38. The second feedbackamplifier 36 receives its input from the output of a fourth summer 40which receives one input directly from the second summer 26 and theoutput of a second time delay circuit 42 also having a time delay ofl/PRF. The input of the time delay circuit 42 is coupled to the outputof the second summer 26.

The digital clutter notch filter is controlled as a function of the PRFsequence to generate a filter characteristic which includes a desiredground target rejection band as well as the ground clutter return foreach PRF. In this regard reference to FIG. 2 illustrates the desiredcharacteristic. As is well known in pulse Doppler radar technology, thefrequency spectrum of the return pulse includes the transmitted spectralline 44 as well as spectral lines 46, 48, 50, etc. which correspond tothe harmonics of the transmitted carrier frequency respectivelyseparated by the instant PRF frequency. Where the radar is mounted on amoving platform, such as a highspeed aircraft, the received groundclutter spectra undergoes a Doppler shift or spread as shown by the vspectral cusps 52 which are symmetrically dispersed around the spectrallines 44, 46, 48 and 50. Any mov ing targets however will display aspectral cusp such as 54 or 55 shifted outside of the ground clutterbecause of the increased Doppler shift due to the relative movementbetween the target and the radar platform. Accordingly, a low speedground target 54 would appear in the spectrum as a cusp 54 adjacent theground clutter spectra whereas a high speed target spectral cusp 55would be shifted even further. It can be seen, therefore, that by properselection of a notch filter characteristic, the reject band can be madeto include the ground clutter spectrum as well as the relatively lowspeed target spectra as well while allowing the desired high speedtarget spectra to fall within the pass band. A detailed description of amultiple delay notch filter such as used in the present invention is setforth in Chapter 9 of Radar Design Principles, Fred E. Nathanson,McGraw- Hill, 1964.

Referring now to FIGS. 3A and 38, there is disclosed in FIG. 3A awideband notch characteristic generated by the filter 16 for five PRFswherein PRF 1 is the lowest PRF utilized while PRF 5 is the highest PRF.It is significant to note that when the wide notch characteristic isselected by means of the switching control circuit 20, a reject bandofil /z normalized frequency units is provided about each PRF harmonicwhich is the desired reject band to delete slow moving targets atrelatively close range. It can be appreciated, however,

'that for the relatively lower PRFs for example PRF I and PRF 2 in FIG.3A, which are utilized for unambiguous detection of targets at largerranges, the ground clutter spectrum cusps 52 shown in FIG. 2 are closelyspaced, thereby restricting the available passband when a wide notch isused to reject the lower speed target return which is undesirable. Sincethe clutter return spectrum for higher PRFs are relatively widelyspaced, the wide notch characteristic is acceptable; however, where longunambiguous range determination is to be made, and the relatively lowerPRFs utilized therefor, it becomes desirable to narrow the reject bandof the notch filter characteristic such as shown in FIG. 38 wherein thereject band of 1% normalized frequency units are utilized and provide acomposite filter characteristic as shown. While the composite filtercharacteristics in both the wide notch and narrow notch mode display abreak in the passband, this can be overcome by the utilization ofadditional PRFs. For this reason, the number of PRFs utilized is shownfor the purposes of illustration only and is not meant to be consideredin a limiting sense. As the PRFs are sequenced, visibility of a highspeed airborne target will be provided by the return appearing in thepassband of at least one of the PRFs. Additionally, a spread of PRFfrequency from four to eight normalized frequency units is utilized toassure good'visibility of the desired targets.

The notch switching time selector l8 and the notch width switchingcontrol circuit in the subject invention operate to control the filtercharacteristics as shown in FIGS. 3A and 38 during the interval betweentransmitted pulses while the PRF sequence occurs. FIG. 4 discloses thatfor a selected one, i.e. the highest PRF, such as PRF l, the wide notchcharacterstic such as shown in FIG. 3A is used. However, for selectedlower PRFs for example PRFs 3, 4 and 5, as shown in FIG. 3B, the narrownotch characteristic is switched thereto after a time during the subjectinterpulse interval which is substantially equal to the period betweenpulses occurring at the highest PRF. Thus rejection notches are switchedduring the interpulse interval for the lower PRFs occurring during atarget dwell time so as to reduce the rejection band as range increases.

FIG. 5 discloses a graph which is illustrative of the system capabilityof a low PRF pulse Doppler radar having but one rejection band notchcharacteristic as opposed to the present invention which has at leasttwo rejection band notch characteristics wherein the wide notchcharacteristic is initially used but the narrow notch characteristic isswitched into the system during the interpulse interval for the low PRFsutilized. FIG. 5 additionally discloses in the dotted line portion ofthe rejection region the capability which would be obtained where fournotch characteristics each of a decreasing notch characteristic isswitched into the system as the PRF-is lowered.

What has been shown and described, therefore, is a low PRF pulse Dopplarradar configured with a plurality of PRFs which are switched during atarget dwell time. The comb or notch filter characteristic associatedwith each PRFis controlled to have a desired Doppler rejection bandwhich includes relatively low speed targets as opposed to desired highspeed targets. Additionally, a notch filter characteristic having arelatively narrower reject band is utilized with lower PRFs. Whenoperating in one of the lower PRFs the wide notch filter is used for atime after pulse transmission corresponding to the unambiguous range ofthe highest PRF, that is the interpulse interval thereof, then thenarrower notch characteristic is used for the remainder of theinterpulse period for the other PRFs. Thus rejection notches areswitched during the interpulse interval so as to reduce the rejectionand as range increases. The significant advantage of the disclosedtechnique is that it approximately doubles the effective unambiguousrange in the radar without affecting performance in the short rangeinterval, the latter being provided by known prior art apparatus. Also,the use of current,

- digital filtering techniques permits the implementation of thedisclosed concept without difficulty or significant complexity.

I claim as my invention:

1. In a pulse Doppler radar system including a transmittcr and receiversection wherein said transmitter section is triggered from a pulserepetition frequency (PRF) generator that produces trigger .pulsescomprised of a plurality of PRFs which are sequentially switched in apredetermined order to produce a plurality of radar output pulses duringeach target dwell time, the improvement comprising:

an MTI filter circuit coupled to the receiver section,

having a filter characteristic which is selectively variable and havingat least two individually selected reject bands for each harmonic ofsaid plurality of PRFs for rejecting both ground clutter return ignalsand selected moving targets; and

means coupled to said PRF generator and being responsive to said triggerpulses, said means being coupled to said filter generating and couplingsignals thereto for controlling the filter characteristic in response toeach PRF and selectively switching the reject band characteristic fromone reject band to another during the sequence of said plurality ofPRFs.

2. The invention as defined in claim 1 wherein said MTI filter comprisesa notch filter and said means switches the filter characteristic fromone reject band to anotherduring the interpulse interval of at least onePRF.

3. The invention as defined by claim 2 wherein said at least one PRFcomprises one of the lower PRFs.

4. The invention as defined by claim 3 wherein said notch filter circuitcomprises a digital notch filter.

5. The invention as defined by claim 1 wherein said MTI filter comprisesa notch filter having a wide notch reject band and a narrow notch rejectband and wherein said means switches said filter characteristic from awide notch reject band characteristic to the nar row notch reject bandcharacteristic during the interpulse interval of at least one of thelower PRFs after a time substantially equal to the interpulse period ofa higher PRF.

6. The invention as defined by claim 5 wherein said means switches thefilter characteristic after a time substantially equal to the interpulseinterval of the highest PRF.

7. The invention as defined by claim 1 wherein said plurality of PRFshave a frequency spectrum of at least four to eight normalized frequencyunits and wherein said at least two reject bands are comprised ofsubstantially :1 A2 normalized frequency units and i'% normalizedfrequency units respectively.

8. The invention as defined by claim 7 wherein said means switches thefilter characteristic during the in terpulse interval of at least one ofthe lower PRFs.

9. The invention as defined by claim 1 wherein said MTI filter comprisesa digital notch filter including:

input circuit means coupled to said receiver section and accepting theIF signal of a radar return signal therefrom;

a first and a second signal summer each having two input ports and oneoutput port and additionally including circuit means coupling said inputcircuit means to one port thereof;

a first time delay circuit coupled between the output port of said firstsignal summer and the other input port of said second signal summer;

a third signal summer having a pair of input ports and one output port;

a second time delay circuit coupled between the output port of saidsecond signal summer and one input port of said third signal summer;

circuit means coupling the output port of said second signal summer tothe other input port of said third signalsummer;

a fourth signal summer having a pair of input ports circuit meanscoupling the output port of said fourth and one output port; signalsummer to the other input port of said first first signal feedbackcircuit means coupled from the signal summer; and

output port of said second signal summer to one output circuit meanscoupled to the output port of input port'of said fourth signal summerand addisaid third signal summer. tionally including circuit meanscoupled to said 10. The invention as defined by claim 9 wherein saidmeans controlling the filter characteristic whereby first and secondtime delay circuits have a time delay the feedback characteristic iscontrolled; substantially equal to the interpulse interval of saidplusecond feedback circuit means coupled from the outrality of pulserepetition frequencies.

put port of said third signal summer to the other 10 11. The inventionas defined by claim 10 wherein input port of said fourth signal summerand includsaid first and second feedback circuit means comprises ingcircuit means coupled to said means for confeedback amplifiers havingthe respective gain charactrolling the filter characteristic whereby thefeedteristic thereof controlled by said means controlling the backcharacteristic of said second feedback'circuit filter characteristic.

means is controlled;' 5

1. In a pulse Doppler radar system including a transmitter and receiversection wherein said transmitter section is triggered from a pulserepetition frequency (PRF) generator that produces trigger pulsescomprised of a plurality of PRFs which are sequentially switched in apredetermined order to produce a plurality of radar output pulses duringeach target dwell time, the improvement comprising: an MTI filtercircuit coupled to the receiver section, having a filter characteristicwhich is selectively variable and having at least two individuallyselected reject bands for each harmonic of said plurality of PRFs forrejecting both ground clutter return ignals and selected moving targets;and means coupled to said PRF generator and being responsive to saidtrigger pulses, said means being coupled to said filter generating andcoupling signals thereto for controlling the filter characteristic inresponse to each PRF and selectively switching the reject bandcharacteristic from one reject band to another during the sequence ofsaid plurality of PRFs.
 2. The invention as defined in claim 1 whereinsaid MTI filter comprises a notch filter and said means switches thefilter characteristic from one reject band to another during theinterpulse interval of at least one PRF.
 3. The invention as defined byclaim 2 wherein said at least One PRF comprises one of the lower PRFs.4. The invention as defined by claim 3 wherein said notch filter circuitcomprises a digital notch filter.
 5. The invention as defined by claim 1wherein said MTI filter comprises a notch filter having a wide notchreject band and a narrow notch reject band and wherein said meansswitches said filter characteristic from a wide notch reject bandcharacteristic to the narrow notch reject band characteristic during theinterpulse interval of at least one of the lower PRFs after a timesubstantially equal to the interpulse period of a higher PRF.
 6. Theinvention as defined by claim 5 wherein said means switches the filtercharacteristic after a time substantially equal to the interpulseinterval of the highest PRF.
 7. The invention as defined by claim 1wherein said plurality of PRFs have a frequency spectrum of at leastfour to eight normalized frequency units and wherein said at least tworeject bands are comprised of substantially + or - 1 1/2 normalizedfrequency units and + or - 3/4 normalized frequency units respectively.8. The invention as defined by claim 7 wherein said means switches thefilter characteristic during the interpulse interval of at least one ofthe lower PRFs.
 9. The invention as defined by claim 1 wherein said MTIfilter comprises a digital notch filter including: input circuit meanscoupled to said receiver section and accepting the IF signal of a radarreturn signal therefrom; a first and a second signal summer each havingtwo input ports and one output port and additionally including circuitmeans coupling said input circuit means to one port thereof; a firsttime delay circuit coupled between the output port of said first signalsummer and the other input port of said second signal summer; a thirdsignal summer having a pair of input ports and one output port; a secondtime delay circuit coupled between the output port of said second signalsummer and one input port of said third signal summer; circuit meanscoupling the output port of said second signal summer to the other inputport of said third signal summer; a fourth signal summer having a pairof input ports and one output port; first signal feedback circuit meanscoupled from the output port of said second signal summer to one inputport of said fourth signal summer and additionally including circuitmeans coupled to said means controlling the filter characteristicwhereby the feedback characteristic is controlled; second feedbackcircuit means coupled from the output port of said third signal summerto the other input port of said fourth signal summer and includingcircuit means coupled to said means for controlling the filtercharacteristic whereby the feedback characteristic of said secondfeedback circuit means is controlled; circuit means coupling the outputport of said fourth signal summer to the other input port of said firstsignal summer; and output circuit means coupled to the output port ofsaid third signal summer.
 10. The invention as defined by claim 9wherein said first and second time delay circuits have a time delaysubstantially equal to the interpulse interval of said plurality ofpulse repetition frequencies.
 11. The invention as defined by claim 10wherein said first and second feedback circuit means comprises feedbackamplifiers having the respective gain characteristic thereof controlledby said means controlling the filter characteristic.